Field of the Invention
The invention relates to a method for identifying short circuits in low-voltage networks using selected switching criteria. In addition, the invention also relates to an associated configurations for carrying out the method.
Short circuits in low-voltage (LV) systems result in high electrodynamic and thermal stresses both on the downstream system parts, such as conductor lines, cables, busbar systems or the like, and on the power breaker or circuit breaker which carries out the disconnection. The amount of stress is governed primarily by the time period from the occurrence of the short circuit until it is cut off. A part of this time is required purely for detecting the unacceptable operating state, this is referred to as the so-called short-circuit identification time. The aim is to find a method which is tolerant to various network parameters, in particular the power factor, and which allows short circuits to be identified quickly.
A range of methods for a short-circuit identification have already been proposed:
Conventionally, the magnitude of the current i is assessed by magnetic and/or thermal releases or triggers, and a disconnection is triggered (i-criterion) if a limit is exceeded. Since the current through the network inductances is continuous, a certain time period always passes before the current rises above the limit and the short circuit is thus identified. A further disadvantage is that it is necessary to set a limit well above (in practice by a factor of .gtoreq.3) the rated root-mean-square current based on the surge factor .kappa., in order to prevent an inadvertent tripping. In consequence, it is possible that "weak" short circuits will never be identified. An additional delay occurs as a result of the mechanical and, in particular, thermal inertia of the releases r trigger devices. In order to compensate for this, numerous electronic releases heave been configured on the basis of an exclusive current assessment (i-criterion), which compare the actual current to the tripping limit with no inertia or with little inertia. PA1 An algorithm is proposed in the reference etz 112 (1991), pages 718 to 722 which, in addition to the current i, also uses the current steepness di/dt for identifying a short circuit. Tripping takes place when the following condition, which is called an extrapolation criterion, is satisfied: ##EQU1## PA1 I.sub.N : is the rated current (root mean square value), PA1 .phi..sub.N : is the phase shift in the rated current circuit, PA1 .tau..sub.N : is the time constant of the rated current circuit with .tau..sub.N =tan (.phi..sub.N)/(2.pi.f), and PA1 G.sub.extra : is the tripping limit. PA1 i: is the current, PA1 di/dt: is the current steepness, PA1 I.sub.N : is a rated current as a root mean square value, PA1 .phi..sub.N : is a phase shift in a rated current circuit, PA1 .tau..sub.N : is a time constant of the rated current circuit where PA1 .tau..sub.N =tan (.phi..sub.N)/(2.pi.f) with f being a network frequency, and G.sub.extra : is a tripping limit. PA1 (dI/dt).sub.2 : is a current steepness at an intersection of two straight extrapolation lines, PA1 I.sub.2 : is a current at the intersection of the two straight extrapolation lines, PA1 I.sub.N : is a rated current as a root mean square value, PA1 .omega.: is a network circular frequency, PA1 .phi..sub.u : is a phase shift between the current and a voltage when using the lower power factor, and PA1 .phi..sub.o : is a phase shift between the current and the voltage when using the upper power factor. PA1 (dI/dt).sub.3 : is the maximum current steepness, PA1 I.sub.3 : is the current at the maximum current steepness, .psi..sub.max : is a switching angle, related to the voltage, with a subsequent maximum current steepness in accordance with ##EQU8## PA1 .phi..sub.u : is a phase shift between the current and a voltage when using the lower power factor, and PA1 .phi..sub.o : is a phase shift between the current and the voltage when using the upper power factor. PA1 first of all, separate envelopes are formed for the lower power factor on the one hand, and the upper power factor on the other hand, with all the switching angles being enclosed in each case, PA1 in addition, a further envelope is determined which takes account of the rated-current switching operations between two power factor limits, PA1 the envelopes obtained in this way are combined and are superimposed to form the resultant envelope which embodies a "Tolerant Locus Curve Criterion" (TLC) which is independent of the power factor and initial current, and a short circuit is indicated if the values are outside the tolerance locus curve criterion.
where
On a graph with current i as the abscissa and the current steepness di/dt as the ordinate, equation (1) defined above represents a straight line.
In comparison with all other known methods, which use the current i and current steepness di/dt for identifying short circuits, the extrapolation criterion is optimal for the identification characteristics such as the identification time, the current heating integral, and the current at the identification time. The extrapolation criterion has a stable reaction with regard to switching the current level within the permissible limits.
A disadvantage of this method is that the power factor in the rated operation of the network to be protected must be known for an adaptation of the method. This requires that the network conditions are known and do not change since, if the load varies, the method will be incorrectly matched and may result in an inadvertent or unauthorized tripping.
A method which is insensitive to different power factors within certain interval limits and likewise uses the current and current steepness for a short-circuit identification is described in the Geriuan patent DE 36 42 136 C1. All the possible combinations of currents and current steepnesses which can occur when a circuit is switched on at power factors of, for example, cos .phi.=0.2 . . . 0.95 are plotted as locus curves in a common diagram, that is to say the current i on the abscissa and the current steepness di/dt on the ordinate. Since, in the German patent DE 36 42 136 C2, all the locus curves start on the ordinate where i is equal to zero, it must be assumed that no current was flowing before the switching operation, and, in consequence, that no initial current is present. Consequently, an envelope is produced around the resultant family of curves, which is declared as a so-called threshold value function. However, no rule for composing the envelope is defined in the German patent DE 36 42 136 C1. If an observation point, which is expressed by the pair of values current/current steepness, leaves the region bounded by the threshold value function, this leads to a tripping.
The German patent DE 36 42 136 C1 cannot take into account those switching processes in which an initial current was flowing, that is to say a so-called changeover takes place, wherein the power factor and/or current change within permissible limits as a result of the switching operation. These operation conditions, which in practice occur with a high probability, can, under certain circumstances and at specific switching phase angles, cause an unjustifiable tripping of the switching device for carrying out the method described in the German patent DE 36 42 136 C1.
Furthermore, the article in the reference ABB Technik 4/1997, page 41 proposes that disconnection criteria for low-voltage switches be developed further through the use of suitable algorithms in order to detect any short circuit which occurs in the microsecond range. The aim is, when a fault occurs in electrical distribution networks with low-voltage systems, to isolate the fault as quickly as possible and to isolate only the faulty part of the system, as well as limiting the down time and the damage to a minimum.